Houston police investigate the scene where the body of missing teenager Vy Ngoc Bao-Pham was found at a Sims Bayou bridge just south of the Gulf Freeway. Her 19-year-old rejected boyfriend is in custody.

A 19-year-old Houston man killed his teenage girlfriend, police said, dumping her body in the Sims Bayou, after she had decided to start seeing someone else.

Houston police identified the boyfriend as Luan Hoang Vu. He is facing a capital murder charge in the death of 17-year-old Vy Ngoc Bao-Pham, whose body was found late Wednesday in a section of the bayou in southeast Houston. Earlier in the day, an Amber Alert had been issued for the girl, who was last seen on Monday.

Vu told police where to find the girl's body, officials said.

"He drew a map to where he put the body. Then he brought us to the bayou and pointed her out," said Fil Waters, a Houston police homicide detective.

Police said Vu wrapped the body in a sheet and slid her into the bayou under a bridge along Broadway. She had been there since Monday.

Vu told police how he killed her, but officials said they are going to wait for autopsy results.

After questioning him at length on Wednesday, police said they were confident of the motive.

"He was obsessed with their relationship," Waters said.

Police said the missing girl had apparently met someone else, however. Vu grabbed her while she was going to visit her new boyfriend, officials said.

Police said he killed her soon after that and took her body to the bayou.

Vu also admitted creating a bogus ransom demand that was sent in a text message to her family, police said. He demanded $50,000 and said they had two weeks to pay.

"He sent the note to deflect what had happened to her," Waters said.

Vu was among the friends and family members who were questioned Tuesday. Waters said he nearly confessed at that time. He refrained because he had heard in his native Vietnam that American police officers were skilled at catching criminals, the detective said.

"He wanted to see if that was so," Waters said. "He came down today (Wednesday) because he wanted to further the test."